To mitigate fatigue due to long-time writing as much as possible, the grip portions of writing instruments have heretofore been designed in various manners. For example, a grip portion of modified shape or increased diameter is known, and the position of a center of balance is ergonomically determined according to the weight of each writing instrument, and a grip portion provided with an anti-slip part are known. As disclosed in Japanese Utility Model Publication No. 57-24522/1982, it is also known that a grip of large diameter and a grip of small diameter are provided so that a user can select a desired grip to be fitted onto a grip portion according to the choice or preference of the user.
However, although a user can change his or her grip position for a writing instrument by gripping the forward end portion, the middle portion or the rearward portion of the writing instrument according to the preference of the user, it is impossible to change a shape of the grip portion of the writing instrument, so that there still remains a problem in that the user feels fatigue after continuous, long-time writing. Further, there is a problem in terms of design since the shape of the conventional grip portion of the writing instrument is fixed and cannot be changed. In the art disclosed in Japanese Utility Model Publication No. 57-24522/1982, although it is possible to provide different grip touches by selecting either one of the grip portions of large diameter and small diameter, the number of selectable grip touches is limited.